U.S. patent number 9,490,479 [Application Number 14/378,930] was granted by the patent office on 2016-11-08 for non-aqueous electrolyte battery.
This patent grant is currently assigned to PANASONIC INTELLECTUAL PROPERTY MANAGEMENT CO., LTD.. The grantee listed for this patent is PANASONIC CORPORATION. Invention is credited to Tadayoshi Takahashi, Toshie Wata.
United States Patent |
9,490,479 |
Wata , et al. |
November 8, 2016 |
Non-aqueous electrolyte battery
Abstract
Provided is a non-aqueous electrolyte battery excellent in
initial static characteristics and continuous charge
characteristics. The non-aqueous electrolyte battery includes a
pellet-shaped positive electrode, a pellet-shaped negative
electrode, a separator interposed between the positive and negative
electrodes, and a non-aqueous electrolyte. The positive electrode
includes a positive electrode active material, aluminum powder, a
conductive agent, and a binder. The positive electrode active
material contains vanadium pentoxide. The positive electrode has a
porosity of 35.6 to 45.4 vol %. The negative electrode includes a
negative electrode active material containing silicon, a conductive
agent, and a binder.
Inventors: |
Wata; Toshie (Osaka,
JP), Takahashi; Tadayoshi (Osaka, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
PANASONIC CORPORATION |
Osaka |
N/A |
JP |
|
|
Assignee: |
PANASONIC INTELLECTUAL PROPERTY
MANAGEMENT CO., LTD. (Osaka, JP)
|
Family
ID: |
49222269 |
Appl.
No.: |
14/378,930 |
Filed: |
March 19, 2013 |
PCT
Filed: |
March 19, 2013 |
PCT No.: |
PCT/JP2013/001871 |
371(c)(1),(2),(4) Date: |
August 14, 2014 |
PCT
Pub. No.: |
WO2013/140791 |
PCT
Pub. Date: |
September 26, 2013 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150017540 A1 |
Jan 15, 2015 |
|
Foreign Application Priority Data
|
|
|
|
|
Mar 22, 2012 [JP] |
|
|
2012-065011 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/13 (20130101); H01M 4/485 (20130101); H01M
4/483 (20130101); H01M 4/131 (20130101); H01M
4/134 (20130101); H01M 4/62 (20130101); H01M
4/386 (20130101); Y02E 60/10 (20130101); H01M
2004/021 (20130101); Y02P 70/50 (20151101) |
Current International
Class: |
H01M
4/13 (20100101); H01M 4/38 (20060101); H01M
4/48 (20100101); H01M 4/62 (20060101); H01M
4/131 (20100101); H01M 4/485 (20100101); H01M
4/134 (20100101); H01M 4/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2007-157704 |
|
Jun 2007 |
|
JP |
|
2009-170428 |
|
Jul 2009 |
|
JP |
|
2007-055276 |
|
May 2007 |
|
WO |
|
Other References
Machine translation of Japanese Patent Document JP 07-226231 A,
published Aug. 22, 1995. cited by examiner .
International Search Report issued in PCT/JP2013/001871, dated Jun.
11, 2013, with English translation. cited by applicant.
|
Primary Examiner: Barcena; Carlos
Assistant Examiner: Nedialkova; Lilia V
Attorney, Agent or Firm: McDermott Will & Emery LLP
Claims
The invention claimed is:
1. A coin-type non-aqueous electrolyte battery comprising a
pellet-shaped positive electrode, a pellet-shaped negative
electrode, a separator interposed between the positive electrode
and the negative electrode, and a non-aqueous electrolyte, the
positive electrode including a positive electrode active material,
aluminum powder, a conductive agent, and a binder, the positive
electrode active material containing vanadium pentoxide, the
positive electrode having a porosity of 36 vol % to 45 vol %, and
the negative electrode including a negative electrode active
material containing silicon, a conductive agent, and a binder.
2. The coin-type non-aqueous electrolyte battery according to claim
1, wherein the positive electrode includes the aluminum powder in
an amount of 1 part by mass to 20 parts by mass, relative to 100
parts by mass of the positive electrode active material.
3. The coin-type non-aqueous electrolyte battery according to claim
1, wherein the negative electrode active material is at least one
selected from the group consisting of silicon alloys and simple
substance of silicon.
4. The coin-type non-aqueous electrolyte battery according to claim
1, wherein the negative electrode active material includes a
silicon-titanium alloy having an amorphous silicon phase.
5. The coin-type non-aqueous electrolyte battery according to claim
1, wherein: the positive electrode active material contains
vanadium pentoxide in the form of particles, and the average
particle size of vanadium pentoxide is 3 .mu.m to 20 .mu.m.
6. The coin-type non-aqueous electrolyte battery according to claim
1, wherein: a percentage of particles having a particle size of 45
.mu.m or less than 45 .mu.m in the aluminum powder as measured by
the Ro-tap method is 60 mass % or more.
7. The coin-type non-aqueous electrolyte battery according to claim
1, wherein the pellet-shaped positive electrode is formed by
compacting in which a material mixture is packed in a molding die
of a predetermined shape, and pressure is applied to the mixture,
such that the positive electrode has a porosity of 36 vol % to 45
vol %.
Description
RELATED APPLICATIONS
This application is the U.S. National Phase under 35 U.S.C.
.sctn.371 of International Application No. PCT/JP2013/001871, filed
on Mar. 19, 2013, which in turn claims the benefit of Japanese
Application No. 2012-065011, filed on Mar. 22, 2013, the
disclosures of which Applications are incorporated by reference
herein.
TECHNICAL FIELD
The present invention relates to a non-aqueous electrolyte battery
which includes a positive electrode including vanadium pentoxide,
and a negative electrode including silicon.
BACKGROUND ART
Non-aqueous electrolyte batteries represented by lithium ion
batteries are widely used as main power source and memory backup
power source in various electronic devices. Particularly in recent
years, the demand for non-aqueous electrolyte batteries shows an
increasing trend year by year, with increase of small portable
devices such as cellular phones, digital still cameras, and
wireless communication devices. While the reduction in size and
weight of the devices proceeds, more functionality and increase in
memory capacity are required for the devices. Therefore, in
applications such as main power source and memory backup power
source, the batteries are required to be small in size and have a
high capacity. In addition, in such applications, it is important
to ensure excellent reliability over a long period of time.
With regard to the positive electrode active material of
non-aqueous electrolyte batteries, for example, manganese dioxide
and graphite fluoride are used in primary batteries; in secondary
batteries, the use of sulfides such as TiS.sub.2 and MoS.sub.2,
oxides such as manganese dioxide and vanadium pentoxide
(V.sub.2O.sub.5), and lithium-containing transition metal composite
oxides such as lithium cobalt oxide, lithium nickel oxide, and
lithium manganese oxide is examined. Vanadium pentoxide can absorb
and release lithium ion, and has a high theoretical capacity.
Non-aqueous electrolyte batteries in which a positive electrode
including vanadium pentoxide is combined with a lithium negative
electrode, due to their small self-discharge, are used for backup
and other purposes.
For example, Patent Literature 1 discloses using a positive
electrode including vanadium pentoxide and aluminum powder, and a
negative electrode including niobium pentoxide doped with lithium,
in lithium secondary batteries used for backup or power supply to
portable devices.
CITATION LIST
Patent Literature
[PTL 1] Japanese Laid-Open Patent Publication No. Hei 7-226231
SUMMARY OF INVENTION
Technical Problem
When used for backup or other similar purposes, the batteries are
very frequently charged, and are likely to fall in an overcharged
state. If the overcharged state continues, the battery
characteristics tend to deteriorate. Therefore, when used for such
purposes, the batteries are required to have excellent continuous
charge characteristics, that is, to show little deterioration in
battery characteristics even after continuous charge.
The charge potential of vanadium pentoxide is high. Therefore, in
an overcharged state, the positive electrode is exposed to an
extremely high potential. As the potential at the positive
electrode becomes higher, vanadium pentoxide is oxidized and
leaches out into the non-aqueous electrolyte, causing a side
reaction at the negative electrode. This facilitates the
deterioration in battery characteristics. In the case where a
battery including a positive electrode including vanadium pentoxide
is used for backup or other similar purposes where the battery is
highly likely to be overcharged, it is particularly desired to
ensure excellent continuous charge characteristics for improving
the battery reliability.
In Patent Literature 1, aluminum powder is added to the positive
electrode including vanadium pentoxide, for the purpose of
suppressing the reduction in capacity of the battery when
overcharged in a high temperature atmosphere. This may be effective
to some extent in improving continuous charge characteristics.
However, in the case of forming a positive electrode into a pellet
shape using vanadium pentoxide, as compared with using other
positive electrode active materials, the porosity tends to
increase. When the positive electrode has a high porosity, the
supply speed of non-aqueous electrolyte to the positive electrode
becomes very fast, which in turn slows the supply speed to the
negative electrode.
In non-aqueous electrolyte batteries, the use of negative
electrodes including graphite and silicon-containing materials is
examined. These negative electrodes can absorb a large amount of
lithium ion, and thus can significantly lower the potential at the
negative electrode. Therefore, they can be effectively used for
obtaining a battery with high capacity. In particular, much
attention is paid to silicon-containing materials, whose lithium
absorption amount is greater than that of graphite.
A higher capacity can be expected with a negative electrode
including a silicon-containing material, since it absorbs a large
amount of lithium ion during charge, thereby to lower the negative
electrode potential. If, however, supply of non-aqueous electrolyte
to the negative electrode becomes insufficient, the negative
electrode potential will not be lowered sufficiently. This results
in a low initial capacity of the battery, and thus in poor initial
static characteristics of the battery. If the negative electrode
potential is in such a state during continuous charge, at the
negative electrode, a side reaction of the conductive agent
included in the negative electrode with lithium and non-aqueous
electrolyte becomes noticeable, the resistance is raised, and the
battery capacity is reduced. In short, the continuous charge
characteristics deteriorate, and thereby the reliability of the
battery degrades. Patent Literature 1 teaches the use of niobium
pentoxide doped with lithium in the negative electrode. The
above-mentioned side reaction involving the conductive agent at the
negative electrode, however, is particularly noticeable in a
negative electrode including a silicon-containing material whose
reduction potential is lower than niobium pentoxide, and the side
reaction leads to poor initial static characteristics and
deterioration in continuous charge characteristics.
In view of the problem as above, one aspect of the present
invention intends to provide a non-aqueous electrolyte battery
which includes a positive electrode including vanadium pentoxide,
and a negative electrode including a silicon-containing material,
and despite this, is excellent in initial static characteristics
and continuous charge characteristics.
Solution to Problem
One aspect of the present invention relates to a non-aqueous
electrolyte battery including a pellet-shaped positive electrode, a
pellet-shaped negative electrode, a separator interposed between
the positive electrode and the negative electrode, and a
non-aqueous electrolyte. The positive electrode includes a positive
electrode active material, aluminum powder, a conductive agent, and
a binder. The positive electrode active material contains vanadium
pentoxide. The positive electrode has a porosity of 35.6 to 45.4
vol %. The negative electrode includes a negative electrode active
material containing silicon, a conductive agent, and a binder.
Advantageous Effects of Invention
According to the above aspect of the present invention, a
non-aqueous electrolyte battery excellent in initial static
characteristics and continuous charge characteristics can be
obtained, although it includes a positive electrode including
vanadium pentoxide, and a negative electrode including a
silicon-containing material.
While the novel features of the invention are set forth
particularly in the appended claims, the invention, both as to
organization and content, will be better understood and
appreciated, along with other objects and features thereof, from
the following detailed description taken in conjunction with the
drawings.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 A schematic cross-sectional view of a coin lithium secondary
battery according to one embodiment of a non-aqueous electrolyte
battery of the present invention
DESCRIPTION OF EMBODIMENT
One aspect of the present invention relates to a non-aqueous
electrolyte battery including a pellet-shaped positive electrode, a
pellet-shaped negative electrode, a separator interposed between
the positive and negative electrodes, and a non-aqueous
electrolyte. In the non-aqueous electrolyte battery, the positive
electrode includes a positive electrode active material, aluminum
powder, a conductive agent, and a binder. The positive electrode
active material contains vanadium pentoxide. The negative electrode
includes a negative electrode active material containing silicon, a
conductive agent, and a binder. In such a non-aqueous electrolyte
battery, the porosity of the positive electrode is 35.6 to 45.4 vol
%.
When a positive electrode is formed into a pellet shape using
vanadium pentoxide as the positive electrode active material,
because of its low packability as compared with other positive
electrode active materials, the porosity of the positive electrode
tends to increase. When the positive electrode is highly porous,
the supply speed of non-aqueous electrolyte to the positive
electrode becomes very fast, which in turn slows the supply speed
to the negative electrode.
In general, in non-aqueous electrolyte batteries in which the
negative electrode includes a negative electrode active material
containing silicon, the absorption reaction of lithium ion into the
negative electrode proceeds very quickly upon injection of
non-aqueous electrolyte, in the process of battery fabrication (in
the initial stage). In other words, when using a negative electrode
including a silicon-containing negative electrode active material,
the non-aqueous electrolyte is considerably consumed at the
negative electrode in the initial stage.
When a positive electrode including vanadium pentoxide is used in
combination with a negative electrode including a
silicon-containing negative electrode active material, the battery
reaction at the negative electrode needs a large amount of
non-aqueous electrolyte in the initial stage. Despite such need,
the supply of non-aqueous electrolyte to the negative electrode
tends to be insufficient, because the positive electrode is likely
to be highly porous. Insufficient supply of non-aqueous electrolyte
to the negative electrode means insufficient absorption of lithium
ion into the negative electrode, which hinders the lowering of
negative electrode potential in the initial stage. Consequently,
the initial static characteristics of the battery tend to be
poor.
Moreover, when the battery is continuously charged while the
negative electrode potential is not sufficiently lowered, at the
negative electrode, the non-aqueous electrolyte is consumed not by
a charge reaction of the silicon-containing negative electrode
active material, but by a side reaction involving the conductive
agent contained in the negative electrode and lithium. The side
reaction is an irreversible reaction. Therefore, due to continuous
charge, the capacity retention rate of the battery is reduced.
When niobium pentoxide as disclosed in Patent Literature 1 is used
as the negative electrode active material, the above problem hardly
occurs since the absorption reaction of lithium into negative
electrode active material occurs preferentially to the
above-mentioned side reaction involving the conductive agent at the
negative electrode. When using a silicon-containing negative
electrode active material, however, the above side reaction becomes
extremely noticeable, and in association therewith, the initial
static characteristics and continuous charge characteristics are
significantly impaired. The reduction in initial static
characteristics and continuous charge characteristics as mentioned
above can be regarded as a problem specific to the combination of a
negative electrode including a silicon-containing negative
electrode active material and a positive electrode including
vanadium pentoxide.
In one embodiment of the present invention, the porosity of the
pellet-shaped positive electrode is controlled to 45.4 vol % or
less. Therefore, even though the positive electrode active material
contains vanadium pentoxide, the speed at which the non-aqueous
electrolyte is absorbed into the positive electrode can be
appropriately slowed, and thus the supply speed of non-aqueous
electrolyte to the negative electrode is not unnecessarily
slowed.
A lower porosity of the positive electrode is considered preferable
because the absorption of non-aqueous electrolyte is slowed, and
the supply of non-aqueous electrolyte to the negative electrode is
unlikely to become insufficient. The present inventors have found,
however, that when the porosity is too low, the battery capacity
reduces during continuous charge. This is presumably for the
following reasons. First, in the process of compressing a mixture
(positive electrode material mixture) containing constituent
components of the positive electrode into a pellet shape, an
unnecessarily large load is applied to the constituent components.
As a result, the particulate components (e.g., positive electrode
active material and conductive agent) collapse, to reduce the
conductivity of the positive electrode. Secondly, during continuous
charge, the side reaction continuously proceeds at the negative
electrode including silicon. This is combined with increases in
internal resistance of the positive and negative electrodes, to
impair the initial static characteristics of the battery. Thirdly,
when the battery is charged continuously while the internal
resistance thereof is high, the battery capacity cannot be fully
utilized, and as a result, the capacity remaining rate after
continuous charge is reduced. In view of the above, the porosity of
the positive electrode should be set to 35.6 vol % or more.
As described above, in one embodiment of the present invention in
which the porosity of the positive electrode is set to 35.6 to 45.4
vol %, despite the combination of a positive electrode including
vanadium pentoxide with a negative electrode including a
silicon-containing negative electrode active material, both the
positive electrode and the negative electrode are allowed to retain
an appropriate amount of non-aqueous electrolyte. It is therefore
possible to suppress the reduction in initial battery static
characteristics, as well as the reduction in capacity during
continuous charge. Therefore, even when used for backup, the
non-aqueous electrolyte battery can exhibit high reliability.
In the following, the configuration of the non-aqueous electrolyte
battery will be more specifically described.
(Positive Electrode)
The pellet-shaped positive electrode has a porosity of 35.6 vol %
or more, preferably 35.8 vol % or more, and more preferably 36 vol
% or more. The porosity of the positive electrode is 45.4 vol % or
less, preferably 45.2 vol % or less, and more preferably 45 vol %
or less. These lower limits and upper limits can be combined in any
combination. For example, the porosity may be 35.8 to 45.4 vol %,
or 36 to 45 vol %.
When the porosity is above 45.4 vol %, the positive electrode
absorbs non-aqueous electrolyte very fast, and therefore,
non-aqueous electrolyte cannot be supplied to the negative
electrode in an amount sufficient for the charge reaction. This
results in an insufficient lowering of negative electrode potential
during charge in the initial stage, and thus in poor initial static
characteristics. Moreover, if the state continues in which the
negative electrode potential is not lowered sufficiently, or
continuous charge is performed in this state, the side reaction
involving the conductive agent included in the negative electrode
occurs at the negative electrode, and the battery capacity is
reduced. That is, the continuous charge characteristics
deteriorate.
When the porosity of the positive electrode is below 35.6 vol %, in
the process of producing a pellet-shaped positive electrode, due to
an application of an unnecessarily large load to the constituent
components of the positive electrode, the particulate constituent
components (e.g., conductive agent and active material particles)
crush or collapse. This breaks the electrically conductive paths
within the positive electrode, reducing the conductivity and
increasing the resistance of the positive electrode in the initial
stage. In other words, the internal resistance of the battery
increases, and thus the initial static characteristics of the
battery become poor. Furthermore, when the battery is charged
continuously while the internal resistance thereof is high, the
capacity retention rate is reduced. In other words, the continuous
charge characteristics also deteriorate, which damages the
long-term reliability of the battery.
The porosity of the positive electrode can be calculated from the
mass, volume and real density of the positive electrode. The real
density of the positive electrode can be calculated from the
contents and specific gravities of the constituent components of
the positive electrode.
The positive electrode active material is not particularly limited
as long as it contains vanadium pentoxide, and may further contain
a known component used as the positive electrode active material
for non-aqueous electrolyte batteries, for example, sulfides such
as TiS.sub.2 and MoS.sub.2; metal oxides such as V.sub.6O.sub.13
and MnO.sub.2; and lithium-containing transition metal composite
oxides such as lithium cobalt oxide, lithium nickel oxide, and
lithium manganese oxide.
The vanadium pentoxide content in the positive electrode active
material is, for example, desirably 70 mass % or more, and may be
80 mass % or more, or 90 mass % or more. The positive electrode
active material may contain vanadium pentoxide only. Specifically,
the vanadium pentoxide content in the positive electrode active
material is 100 mass % or less. These lower limits and upper limits
can be combined in any combination. By using a positive electrode
active material containing vanadium pentoxide, a flat high-voltage
profile can be readily obtained, and a battery with high capacity
can be readily realized. The self-discharge of the battery is
small, and an enhanced reliability can be readily achieved even
when the battery is used for backup.
Vanadium pentoxide is usually in the form of particles. The average
particle size of vanadium pentoxide is, for example, 1 to 30 .mu.m,
preferably 3 to 20 .mu.m, and more preferably, 5 to 15 .mu.m. The
average particle size can be determined, for example, on the basis
of a specific surface area calculated by an air permeability
method.
By using aluminum powder in combination with vanadium pentoxide,
the vanadium pentoxide is unlikely to be oxidized and leach out
during overcharge. If an oxide of vanadium pentoxide leaches out, a
surface film is formed on the negative electrode, to inhibit the
charge and discharge reaction, which reduces the capacity. Although
the reason is unclear, the addition of aluminum powder to the
positive electrode is considered to permit the oxidation of
aluminum to occur preferentially to the oxidation of vanadium
pentoxide, even when the potential at the positive electrode
becomes excessively high during overcharge. The oxide of aluminum
thus produced exists stably within the positive electrode.
Therefore, the addition of aluminum powder to the positive
electrode can suppress the reduction in capacity during
overcharge.
Although there is no limitation on the particle size of aluminum
powder, the smaller the better so that aluminum powder can be
readily oxidized during overcharge. For example, the percentage of
particles having a particle size of 45 .mu.m or less in the
aluminum powder as measured by the Ro-tap method is preferably 60
mass % or more, and more preferably 70 mass % or more, or 80 mass %
or more.
The amount of the aluminum powder included in the positive
electrode is, for example, 1 to 20 parts by mass, preferably 1 to
10 parts by mass, and more preferably 1.5 to 8 parts by mass,
relative to 100 parts by mass of the positive electrode active
material. When the amount of aluminum powder is within the range as
above, the reduction in battery capacity during overcharge can be
more effectively suppressed.
The conductive agent included in the positive electrode may be any
electrically conductive material that is stable (e.g., causes no
chemical reaction) within the operating potential range during
charge and discharge. Examples of the conductive agent include:
graphites, such as natural graphite and artificial graphite; carbon
blacks, such as acetylene black and Ketjen black; conductive
fibers, such as carbon fibers and metal fibers; and fluorinated
carbon. These conducive agents may be used singly or in combination
of two or more. For ease of ensuring good conductivity at the
positive electrode, it is preferable to use carbon blacks such as
Ketjen black as the conductive agent.
The amount of conductive agent is, for example, 1 to 30 parts by
mass, preferably 2 to 20 parts by mass, and more preferably 3 to 10
parts by mass, relative to 100 parts by mass of the positive
electrode active material. When the amount of conductive agent is
within the range as above, at the positive electrode, good
conductivity can be easily ensured, and the increase in internal
resistance in the initial stage can be easily suppressed.
Examples of the binder included in the positive electrode include:
polyolefins, such as polyethylene and polypropylene; fluorocarbon
resins, such as polytetrafluoroethylene, polyvinylidene fluoride,
tetrafluoroethylene-hexafluoropropylene copolymer, and modified
products thereof; rubbery polymers, such as styrene butadiene
rubber and modified acrylonitrile rubber; and acrylic polymers,
such as polyacrylic acid, acrylic acid-methacrylic acid copolymer,
and salts thereof (e.g., sodium salts and ammonium salts). These
binders can be used singly or in combination of two or more. Among
these binders, for ease of ensuring the strength of the electrode,
fluorocarbon resins such as tetrafluoroethylene-hexafluoropropylene
copolymer are particularly preferred.
The amount of binder is, for example, 0.5 to 10 parts by mass,
preferably 1 to 8 parts by mass, and more preferably 1.5 to 7 parts
by mass, relative to 100 parts by mass of the positive electrode
active material. When the amount of binder is within the range as
above, good conductivity of the positive electrode can be easily
ensured, while the strength thereof is ensured.
The positive electrode includes, as essential components, a
positive electrode active material, aluminum powder, a conductive
agent, and a binder. The positive electrode can be produced by
preparing a mixture (material mixture) including these components,
and compacting the material mixture into a pellet shape. By
adjusting the pressure applied to the material mixture when
compacted, the porosity can be adjusted. The compacting can be
performed by, for example, packing the material mixture in a
molding die of a predetermined shape, and applying pressure to the
mixture. The material mixture compacted into a pellet shape may be
dried by air or heat under reduced or atmospheric pressure. The
porosity of the positive electrode can be indirectly controlled by
adjusting the mass and volume of the pellet. The size of the
positive electrode pellet (diameter, when the pellet is disk-like)
is usually fixed. Therefore, by adjusting the mass and thickness,
the pressure applied to the material mixture to be compressed can
be adjusted, and a positive electrode pellet having a desired
porosity can be obtained. The positive electrode may be provided
with a known current collector, if necessary.
A dispersion medium may be used for dispersing the material
mixture, if necessary. Examples of the dispersion medium include:
water, alcohols such as ethanol, ethers such as tetrahydrofuran,
amides such as dimethylformamide, N-methyl-2-pyrrolidone, and mixed
solvents thereof. The binder may be used in the form of dispersion
in which the binder is dispersed in the dispersion medium.
(Negative Electrode)
The negative electrode includes a negative electrode active
material containing silicon (or a silicon-containing material), a
conductive agent, and a binder.
Examples of the silicon-containing material include simple
substance of silicon, silicon alloys, and silicon compounds (e.g.,
nitrides, sulfides, and oxides). These silicon-containing materials
may be used singly or in combination of two or more. Preferred
among them are silicon alloys and simple substance of silicon.
An example of the silicon alloys is an alloy of silicon and a
transition metal.
In the silicon alloy, an electrochemically active amorphous silicon
phase (amorphous Si phase) and an electrochemically inactive phase
may coexist. The inactive phase functions to lessen the stress of
expansion and contraction of the amorphous Si phase associated with
charge and discharge, as well as to make the negative electrode
active material electrically conductive. Such an electrochemically
inactive phase includes an intermetallic compound of silicon and a
transition metal element constituting the alloy. The amorphous Si
phase sometimes includes very small crystallites, the size of which
is, however, too small to be observed on X-ray diffraction spectra,
and is, for example, 10 nm or less.
Examples of the transition metal element constituting the silicon
alloy include: Group 4 elements in the periodic table such as Ti
and Zr; Group 6 elements such as Cr, Mo and W; Group 7 elements
such as Mn; Group 8 elements such as Fe; Group 9 elements such as
Co; Group 10 elements such as Ni; and Group 11 elements such as Cu.
The intermetallic compound may contain one or two transition metal
elements. Preferable examples among them include Ti, Ni, W and Co.
Specifically, a preferable silicon alloy is, for example, Si--Ti
alloy, Si--Ni alloy, Si--W alloy, and Si--Co alloy. For ease of
ensuring good conductivity, the negative electrode active material
preferably includes Si--Ti alloy, and particularly preferably
includes a Si-titanium alloy having an amorphous Si phase.
In the silicon alloy, the mass ratio of the silicon to the
transition metal element (silicon:transition metal element) is, for
example, 40:60 to 80:20, preferably 50:50 to 75:25, and more
preferably 55:45 to 70:30. When the mass ratio of the silicon to
the transition metal element is within the range as above, a high
capacity of the battery can be easily ensured. In addition, stress
associated with changes in volume of the active material during
charge and discharge can be easily lessened, and good conductivity
can be easily ensured. Note that the mass ratio of the silicon to
the transition metal element can be considered equally to the mass
ratio of the silicon to the transition metal used for forming an
alloy.
The silicon alloy can be produced by any known method, such as
mechanical alloying, vacuum vapor deposition, plating, gas-phase
chemical reaction, liquid quenching, and ion beam sputtering.
The negative electrode active material may be doped with lithium in
advance. Lithium doping can be accomplished by, for example, upon
production of a pellet-shaped negative electrode (or negative
electrode precursor) including a negative electrode active
material, immersing the negative electrode precursor with lithium
foil laminated thereon in non-aqueous electrolyte, to cause an
electrochemical short circuit. Lithium doping is preferably
performed in the process of battery fabrication, in view of
suppressing the increase in internal resistance and the reduction
in capacity of the negative electrode.
The conductive agent included in the negative electrode can be
selected from those exemplified for the positive electrode, and is
preferably a carbonaceous one. Among conductive agents, graphite is
more preferred because of its low volume and good conductivity.
The amount of conductive agent is, for example, 15 to 45 parts by
mass, preferably 18 to 42 parts by mass, and more preferably 20 to
40 parts by mass, relative to 100 parts by mass of the negative
electrode active material. When the amount of conductive agent is
within the range as above, good conductivity can be easily ensured,
and the reduction in battery capacity can be more effectively
suppressed even during continuous charge.
The binder included in the negative electrode can be selected from
those exemplified for the positive electrode. For ease of obtaining
high bonding and favorable battery characteristics, an acrylic
polymer or a salt thereof is preferred. The acrylic polymer is, for
example, a polymer which contains, as a monomer unit, at least one
selected from acrylic acid and methacrylic acid. Specific examples
of the acrylic polymer include: polyacrylic acid, poly methacrylic
acid, acrylic acid-methacrylic acid copolymer, copolymers of
acrylic acid and/or methacrylic acid with another copolymerizable
monomer (e.g., olefine, acrylic acid ester, and methacrylic acid
ester) such as ethylene-acrylic acid copolymer and acrylic
acid-methyl acrylate copolymer, and salts thereof (e.g., alkali
metal salts such as sodium salts; and ammonium salts). Preferred
among them are polyacrylic acid.
The binders may be used singly or in combination of two or more.
The binder may be used in the form of dispersion in which the
binder is dispersed in a dispersion medium.
The amount of binder is, for example, 1 to 20 parts by mass, and
preferably 5 to 15 parts by mass, relative to 100 parts by mass of
the negative electrode active material.
The negative electrode includes, as essential components, a
negative electrode active material containing silicon, a binder,
and a conductive agent. Like the positive electrode, the negative
electrode can be produced by preparing a mixture (material mixture)
including these components, and compacting the material mixture
into a pellet shape. The negative electrode may be provided with a
known current collector, if necessary.
(Separator)
The pellet-shaped positive electrode and the pellet-shaped negative
electrode are arranged so as to face each other with a separator
interposed therebetween.
The separator may be, for example, in the form of woven or
non-woven fabric, or a polyolefin microporous film.
Examples of a resin constituting the woven or non-woven fabric
include: polyolefins, such as polypropylene; polyphenylene
sulfides; aromatic polyamides, such as aramid; polyimide resins,
such as polyimide and polyamide-imide; and polyether ether ketones.
The woven or non-woven fabric may contain one or two or more of
these resins. The polyolefin contained in the microporous film is,
for example, polyethylene, polypropylene, or ethylene-propylene
copolymer.
The separator may be of any shape and size, as long as it can
electrically insulate the positive electrode from the negative
electrode. For example, the separator used with disk-like positive
and negative electrodes is of a circular shape having a size
somewhat larger than each of the areas facing the positive
electrode and the negative electrode.
The thickness of the separator can be selected as appropriate from
the range of, for example, 10 to 250 .mu.m.
(Non-Aqueous Electrolyte)
The non-aqueous electrolyte includes a non-aqueous solvent, and a
lithium salt dissolving in the non-aqueous solvent. Any known
non-aqueous solvent and lithium salt may be used without
limitation.
Non-limiting examples of the non-aqueous solvent include: cyclic
carbonates, such as ethylene carbonate (EC), propylene carbonate
(PC), and butylene carbonate; chain carbonates, such as dimethyl
carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic
ethers, such as 1,4-dioxane, 1,3-dioxolane, tetrahydrofuran,
2-methyltetrahydrofuran, and 3-methyltetrahydrofuran; chain ethers,
such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane,
1,3-dimethoxypropane, diethylene glycol dimethyl ether, and
tetraglyme; lactones, such as .gamma.-butyrolactone; and sulfoxide
compounds, such as sulfolane. These non-aqueous solvents may be
used singly or in combination of two or more.
Non-limiting examples of the lithium salt include: lithium salts of
fluorine-containing acid imides, such as
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2, and
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2); lithium salts of
fluorine-containing acids, such as LiPF.sub.6, LiBF.sub.4, and
LiCF.sub.3SO.sub.3; and lithium salts of chlorine-containing acids,
such as LiClO.sub.4. These lithium salts may be used singly or in
combination of two or more. The lithium salt concentration in the
non-aqueous electrolyte is, for example, 0.5 to 2 mol/L.
The non-aqueous electrolyte may contain, if necessary, a known
additive which is, for example, a carbonate having a polymerizable
unsaturated bond, such as vinylene carbonate and vinylethylene
carbonate, and an aromatic compound, such as cyclohexylbenzene and
diphenyl ether.
The non-aqueous electrolyte may be a solution in which the lithium
salt is dissolved in the non-aqueous solvent, or a gel in which the
solution is retained in a polymeric material. Examples of the
polymeric material include: fluorocarbon resins, such as
polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene
copolymer; chlorine-containing vinyl resins, such as polyvinyl
chloride; vinyl cyanide resins, such as polyacrylonitrile; acrylic
resins, such as polyacrylates; and polyalkylene oxides, such as
polyethylene oxide. These polymeric materials may be used singly or
in combination of two or more.
The non-aqueous electrolyte battery can be fabricated by, for
example, placing pellet-shaped positive and negative electrodes, a
separator interposed therebetween, and the non-aqueous electrolyte
in a battery case, and sealing the case with a sealing plate. The
fabrication method is not particularly limited, and any known
method can be employed. To fabricate a coin non-aqueous electrolyte
battery, for example, the positive electrode is placed in a battery
case (on the inner bottom surface, for example), then the separator
is placed on the positive electrode, and the non-aqueous
electrolyte is injected into the case. Subsequently, a sealing
plate with the negative electrode attached onto the inner surface
thereof is fitted to the opening of the battery case via a gasket,
thereby to seal the case. A coin lithium secondary battery can be
thus obtained.
FIG. 1 is a schematic cross-sectional view of a coin lithium
secondary battery according to one embodiment of the present
invention.
A coin lithium secondary battery 10 includes a disk-like pellet of
positive electrode 4, a disk-like pellet of negative electrode 5, a
separator 6 interposed between the positive and negative electrodes
4 and 5, and a non-aqueous electrolyte (not shown). The positive
electrode 4 includes a vanadium pentoxide-containing positive
electrode active material, aluminum powder, a conductive agent, and
a binder, and has a porosity of 35.6 to 45.4 vol %. The negative
electrode 5 includes a silicon-containing negative electrode active
material, a conductive agent, and a binder.
The separator 6 is resin nonwoven fabric or microporous film cut
out into a circular shape. The positive electrode 4 and the
negative electrode 5, which are electrically insulated from each
other via the separator 6, are placed in a battery case 1 made of
stainless steel, such that the positive electrode 4 comes in
contact with the inner bottom surface of the battery case 1.
The battery case 1 is provided, on its inner side wall from the
opening, with an ejection-molded ring-shaped gasket 3 made of resin
(e.g., polypropylene). A portion at the opening upper end of the
battery case 1 is curled inward by crimping, with the gasket 3
interposed between the portion and a sealing plate 2 made of
stainless steel. The battery case 1 and the sealing plate 2
constitute a battery housing.
It is to be noted that the negative electrode may be doped with
lithium in the process of battery fabrication. Specifically,
lithium foil is laminated on a disk-like pellet of negative
electrode (or negative electrode precursor) including a negative
electrode active material, a conductive agent, and a binder. The
negative electrode with lithium foil laminated thereon, a positive
electrode, and a separator interposed therebetween are placed in a
battery case, into which non-aqueous electrolyte is then injected.
In that way, by electrochemically shorting the negative electrode
precursor laminated with lithium foil by immersing it in the
non-aqueous electrolyte, a negative electrode in which the negative
electrode active material is doped with lithium can be
obtained.
EXAMPLES
The present invention will now be specifically described with
reference to Examples and Comparative Examples. It is to be noted,
however, the present invention should not be construed as limited
to the following Examples.
Example 1
A coin lithium secondary battery A1 as illustrated in FIG. 1 was
fabricated in the manner described below.
(1) Preparation of Negative Electrode Precursor
Ti--Si alloy was synthesized by mechanical alloying. Specifically,
Ti and Si were placed in a mass ratio of 35:65 in a vibratory ball
mill, together with 15-mm-diameter stainless steel balls. The
atmosphere in the mill was replaced with argon, and maintained at 1
atm. Mechanical alloying was performed under these conditions for
80 hours, with the vibratory ball mill being driven at an amplitude
of 8 mm and the number of revolutions of 1200 rpm. The resultant
alloy powder was classified, to obtain alloy powder having a
particle size of 50 .mu.m or less as a negative electrode active
material.
The negative electrode active material, graphite serving as a
conductive agent, and polyacrylic acid serving as a binder were
mixed in a solid content mass ratio of 100:30:10, to give a
negative electrode material mixture. The negative electrode
material mixture was formed into a disk-like pellet having a
diameter of 7.0 mm and a thickness of 0.30 mm, and dried at
160.degree. C. for 12 hours, to prepare a negative electrode
precursor.
The binder used here was an aqueous solution of non-crosslinked
polyacrylic acid having a weight average molecular weight of
1,000,000 (available from Toagosei Co., Ltd.). The conductive agent
used here was graphite having an average particle size of 10 .mu.m
(available from Nippon Graphite Industries Ltd.).
(2) Production of Positive Electrode
Vanadium pentoxide (average particle size as measured by air
permeability method: 8 .mu.m) serving as a positive electrode
active material, Ketjen black serving as a conductive agent, an
aqueous dispersion of a fluorocarbon resin serving as a binder, and
aluminum powder were mixed in a solid content mass ratio of
87:7:3:3, to give a positive electrode material mixture. The
positive electrode material mixture was compressed into a disk-like
pellet having a diameter of 6.2 mm and a thickness of 1.09 mm, and
dried at 200.degree. C. for 10 hours, to form a positive electrode.
The aluminum powder used here included particles having a size of
45 .mu.m or less as measured by the Ro-tap method in an amount of
80 mass % or more.
(3) Fabrication of Battery
A coin lithium secondary battery as illustrated in FIG. 1 was
fabricated in the manner described hereinearlier.
In the battery fabrication, metal lithium foil was laminated on the
negative electrode precursor prepared in (1), and the precursor
laminated with lithium foil was electrochemically shorted within
the battery by bringing it into contact with the non-aqueous
electrolyte, thereby to allow Si in the precursor to be alloyed
with lithium. The negative electrode was thus formed.
The battery had an outside diameter of 9.5 mm and an overall height
of 2.0 mm. The battery fabricated through the above process was
denoted as battery A1. In the manner similar to the above, 10
batteries A1 were fabricated in total.
The separator used here was a non-woven fabric made of
polypropylene. The gasket used here was made of polypropylene. The
non-aqueous electrolyte used here was prepared by dissolving
LiBF.sub.4 serving as a lithium salt in a PC:EC:DME=1:1:1 (volume
ratio) mixed solvent serving as a non-aqueous solvent. The lithium
salt concentration in the non-aqueous electrolyte was 1 mol/L. The
amount of non-aqueous electrolyte injected into the battery was 45
.mu.l.
(4) Evaluation
The resultant batteries and the positive electrodes used for the
batteries were evaluated for the following items (a) to (c).
(a) Porosity of Positive Electrode
The fabricated batteries were disassembled, and the positive
electrodes were taken out, and then washed and dried. The mass and
dimensions (volume) of each positive electrode were measured. The
true density of each positive electrode was determined based on the
specific gravities and amounts of constituent components of the
positive electrode. From the mass, dimensions and true density thus
determined, the porosity of the positive electrode was
calculated.
(b) Evaluation of Series Resistance (IR)
The batteries upon fabrication were aged by heating at 45.degree.
C. for 72 hours. The series resistance (IR) of each battery after
aging was measured, and an average of the measured values of 10
batteries was calculated. In the measurement, a series resistance
across the positive terminal (positive electrode case of the
battery) and the negative terminal (negative electrode sealing
plate) was measured with a resistance meter by a sine wave AC
method (1 kHz).
(c) Evaluation of Continuous Charge Characteristics
The batteries upon fabrication were aged by heating at 45.degree.
C. for 72 hours, and subsequently, charged at a constant voltage of
3.7 V for approximately 40 hours via a 510.OMEGA. resistance. The
charged batteries were discharged via a 10 k.OMEGA. resistance
until the battery voltage reached 2.0 V. The discharge capacity
(initial discharge capacity) when the battery voltage reached 2.0 V
was measured, and an average of the measured values of 10 batteries
was calculated.
Subsequently, the batteries were continuously charged in a
60.degree. C. atmosphere. The discharge capacity after continuous
charge was measured in the same way as measuring the initial
discharge capacity, and an average value was calculated. The
continuous charge was performed by continuously applying a 3.7 V
voltage for 100 days. The ratio (%) of the average value of the
discharge capacity after continuous charge of the battery to that
of the initial discharge capacity was calculated as a capacity
remaining rate (or capacity retention rate) after continuous
charge, which was used as an indicator of the continuous charge
characteristics. The higher the capacity remaining rate is, the
more excellent continuous charge characteristics it indicates.
Examples 2 and 3 and Comparative Examples 1 to 4
Positive electrodes were produced in the same manner as in Example
1, except that in (2) Production of positive electrode the porosity
was adjusted by changing the thickness of the pellet. Batteries A2
to A7 were fabricated and evaluations were conducted in the same
manner as in Example 1, except for using the positive electrodes
thus produced. The thickness of the positive electrode pellet in
each battery was adjusted as follows: 1.21 mm (battery A2), 1.19 mm
(battery A3), 1.17 mm (battery A4), 1.01 mm (battery A5), 1.00 mm
(battery A6), and 0.97 mm (battery A7).
The results of Examples and Comparative Examples are shown in Table
1. Here, batteries A1, A4 and A5 are of Examples, and batteries A2,
A3, A6 and A7 are of Comparative Examples.
TABLE-US-00001 TABLE 1 Porosity of Series Capacity positive
electrode resistance remaining rate Battery (vol %) (.OMEGA.) (%)
A2 47 33 11 A3 46 32 21 A4 45 30 70 A1 41 32 69 A5 36 29 70 A6 35
90 18 A7 34 134 9
Table 1 shows that batteries A1, A4 and A5 in which the porosity of
the positive electrode was within the range of 35.6 to 45.4 vol %
exhibited a very high capacity remaining rate even after continuous
charge at 60.degree. C. In addition, the initial series resistances
of these batteries were low. This is presumably because the
reduction in conductivity at the positive electrode in the initial
stage was suppressed.
In contrast, in batteries A2 and A3 in which the porosity of the
positive electrode was 46 vol % or more, the initial series
resistance was low, but the capacity remaining rate after
continuous charge was significantly reduced. Presumably, in these
batteries, the positive electrode absorbed non-aqueous electrolyte
very fast, which in turn caused insufficient supply of non-aqueous
electrolyte to the negative electrode during charge. As a result,
the charge reaction at the negative electrode failed to proceed
sufficiently, and thus the negative electrode potential was not
lowered sufficiently. If continuous charge is performed in such a
state, the side reaction involving the conductive agent readily
proceeds at the negative electrode. Presumably because of this, the
battery capacity after continuous charge was significantly
reduced.
In batteries A6 and A7, too, in which the porosity of the positive
electrode was 35 vol % or less, the capacity remaining rate after
continuous charge was significantly reduced. This is presumably
because, in the process of forming a positive electrode pellet, the
constituent components were subjected to an unnecessarily high
pressure, and, due to the pressure, the constituent components in
the particulate state collapsed, failing to obtain sufficient
conductivity. Moreover, presumably, the battery was continuously
charged while the internal resistance thereof was high, which
resulted in a significant reduction in capacity.
Although the present invention has been described in terms of the
presently preferred embodiments, it is to be understood that such
disclosure is not to be interpreted as limiting. Various
alterations and modifications will no doubt become apparent to
those skilled in the art to which the present invention pertains,
after having read the above disclosure. Accordingly, it is intended
that the appended claims be interpreted as covering all alterations
and modifications as fall within the true spirit and scope of the
invention.
INDUSTRIAL APPLICABILITY
The non-aqueous electrolyte battery according to one embodiment of
the present invention is excellent in continuous charge
characteristics. It is also excellent in initial static
characteristics. In short, the non-aqueous electrolyte battery can
stably exhibit excellent battery characteristics. Therefore, the
non-aqueous electrolyte battery can be used as main power source or
backup power source for various applications including small
portable devices such as cellular phones and digital still
cameras.
REFERENCE SIGNS LIST
1 Battery case 2 Sealing plate 3 Gasket 4 Positive electrode 5
Negative electrode 6 Separator 10 Coin lithium secondary
battery
* * * * *